Thin film deposition techniques are widely used to form layers of specific materials in the fabrication of semiconductor devices and other micro-devices. The size of the individual components in the devices is constantly decreasing, and the number of layers in the devices is increasing. As a result, the density of components and the aspect ratios of depressions (e.g., the ratio of the depth to the size of the opening) are increasing. The size of workpieces is also increasing to provide more real estate for forming more dies (i.e., chips) on a single workpiece. Many fabricators, for example, are transitioning from 200 mm to 300 mm workpieces, and even larger workpieces will likely be used in the future. Thin film deposition techniques accordingly strive to produce highly uniform layers that cover the sidewalls, bottoms, and corners in deep depressions that have very small openings.
One widely used thin film deposition technique is Chemical Vapor Deposition (CVD). In a CVD system, one or more precursors that are capable of reacting to form a solid thin film are mixed in a gas or vapor state, and then the precursor mixture is presented to the surface of the workpiece. CVD systems can be continuous flow systems or pulsed-type CVD systems. Continuous flow systems provide a continuous flow of the precursor gases. Pulsed-CVD systems intermittently pulse a mixture of the precursor gases between pulses of a purge gas. The surface of the workpiece catalyzes the reaction between the precursor gases to form a thin, solid film at the workpiece surface. A common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction.
Although CVD techniques are useful in many applications, they also have several drawbacks. For example, if the precursors are not highly reactive, then a high workpiece temperature is needed to achieve a reasonable deposition rate. Such high temperatures are not typically desirable because heating the workpiece can be detrimental to the structures and other materials already formed on the workpiece. Implanted or doped materials, for example, can migrate within the silicon substrate at higher temperatures. On the other hand, if more reactive precursors are used so that the workpiece temperature can be lower, then reactions may occur prematurely in the gas phase before reaching the substrate. This is undesirable because the film quality and uniformity may suffer, and also because it limits the types of precursors that can be used.
Atomic Layer Deposition (ALD) is another thin film deposition technique. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer of gas molecules Ax coats the surface of a workpiece W. The layer of Ax molecules is formed by exposing the workpiece W to a precursor gas containing Ax molecules, and then purging the chamber with a purge gas to remove excess Ax molecules. This process can form a monolayer of Ax molecules on the surface of the workpiece W because the Ax molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The layer of Ax molecules is then exposed to another precursor gas containing By molecules. The Ax molecules react with the By molecules to form an extremely thin layer of solid material on the workpiece W. The chamber is then purged again with a purge gas to remove excess By molecules.
FIG. 2 illustrates the stages of one cycle for forming a thin solid layer using ALD techniques. A typical cycle includes (a) exposing the workpiece to the first precursor Ax, (b) purging excess Ax molecules, (c) exposing the workpiece to the second precursor By, and then (d) purging excess By molecules. In actual processing, several cycles are repeated to build a thin film on a workpiece having the desired thickness. For example, each cycle may form a layer having a thickness of approximately 0.5-1.0 Å, and thus it takes many cycles to form a solid layer having a thickness of approximately 60 Å.
FIG. 3 schematically illustrates an ALD reactor 10 having a reaction chamber 20 coupled to a gas supply 30 and a vacuum 40. The reactor 10 also includes a heater 50 that supports the workpiece W and a gas dispenser 60 in the reaction chamber 20. The gas dispenser 60 includes a plenum 62 operatively coupled to the gas supply 30 and a distributor plate 70 having a plurality of holes 72. The plenum 62 is an open compartment through which the gases pass directly to the holes 72. In operation, the heater 50 heats the workpiece W to a desired temperature, and the gas supply 30 selectively injects the first precursor Ax, the purge gas, and the second precursor By as described above for ALD or CVD processes. The vacuum 40 maintains a negative pressure in the chamber to draw the gases from the gas dispenser 60, across the workpiece W, and then through an outlet of the reaction chamber 20.
One concern of both the CVD and ALD processes is providing a consistent deposition rate, precise composition, and uniform thickness across the workpiece. These factors are influenced by the parameters of the individual gas flows of the constituent gases. For example, variances in the gas pressures, valve timing and response times can result in inconsistent volumes of precursor gases being dispensed from the gas dispenser. Conventional Pulsed-CVD processes and ALD processes attempt to provide consistent volumes of precursor gases by using more precise valves or mass flow controllers (i.e., flow meters). Although such devices are an improvement, they may not provide sufficiently accurate amounts of precursor gases to the workpiece.
Another concern associated primarily with ALD processing is that it has a relatively low throughput compared to CVD techniques. For example, ALD processing may take several seconds to perform each Ax-purge-By-purge. This results in a total process time of several minutes to form a single layer less than 100 Å thick. In contrast to ALD processing, CVD techniques require much less time to form a layer of the same thickness. The low throughput of existing ALD techniques limits the utility of the technology in its current state because ALD may be a bottleneck in the overall manufacturing process. Thus, it would be useful to increase the throughput of ALD techniques so that they could be used in a wider range of applications.